In the United States, the standard showerhead has a ½ inch female NPT thread.
In the United States, the Energy Policy Act of 1992 set a limit of 2.5 gallons per minute. In 2010, the maximum permitted flow rate was reduced to 2 gallons per minute (7.6 liters per minute).
American Society of Mechanical Engineers, ASME A112.18.1-2018
Canadian Standards Association, CSA B125.1
“Plumbing Supply Fittings”
The Fed's rulings and reasoning can be examined at:
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Last revised: 20 January 2013.